PROJECT SUMMARY The development of compensatory behavioral strategies that circumvent impairments to enable life activities is a predictable response to the onset of functional disability. Increased reliance on the less-affected, nonparetic, hand and arm for the performance of daily activities after stroke is a prominent example due to the prevalence of stroke and of upper limb hemiparesis as a chronic post-stroke impairment modality. This compensatory strategy involves major behavioral changes which start early after stroke and have strong potential to impact post-stroke neural reorganization patterns via mechanisms of experience-dependent plasticity. The focus of this project across periods has been on understanding neural changes that are driven by compensatory experiences of the nonparetic forelimb and their impact for functional outcome, as studied in rodent models of chronic upper extremity impairments resulting from primary motor cortical (M1) infarcts. We have previously established that learning to rely on the nonparetic forelimb interacts with post-ischemic regenerative responses in both hemispheres to influence behavioral function bilaterally. Its impact for the paretic side is to exacerbate disuse and lessen the efficacy of motor rehabilitative training of the paretic limb. In the most recent project period, we have firmly linked these deleterious effects of learning with the nonparetic forelimb to its disruption of functionally relevant reorganization of peri-infarct M1 and its promotion of synaptic changes in the same region. We have also uncovered that learning compensatory ways of using the two limbs together is beneficial for unimanual function of the paretic side. These and related findings lead us to hypothesize that unimanual experiences of the nonparetic forelimb promote synaptic changes that compete, whereas bimanual experiences promote those that cooperate, with patterns of synaptic change mediating improved paretic forelimb function. We lack detailed knowledge of the time course of the synaptic changes that are promoted by experiences of either forelimb and clear evidence that they compete, and how they vary with bimanual experiences is untested. One major goal for this project is to efficiently tackle these knowledge gaps by capitalizing on in vivo imaging approaches to monitor synaptic structural responses to forelimb experiences in peri-infarct cortex as they unfold over time. Another is to probe the efficacy of bimanual skill training as a strategy for competing with and countering deleterious influences of compensating with the nonparetic side. The specific aims are to test the hypotheses that: (Aim 1) skill learning with the nonparetic forelimb shapes synaptic connectivity in peri-infarct M1 via the activity of contralesional cortex and transcallosal projections to diminish its responsiveness to rehabilitative training of the paretic forelimb, (Aim 2) the same pathway mediates synaptic structural responses in peri-infarct cortex to bimanual skill training that improves paretic forelimb function and (Aim 3) bimanual skills have the capacity to compete with and overcome maladaptive behavioral and synaptic connectivity patterns resulting from unimanual experiences of the nonparetic forelimb.